With the reduction in availability of traditional petroleum sources for liquid fuels, increased activity has occurred in the processing of solid fuel sources such as various rankings of coal. Attempts have been made to provide a commercially attractive process for the production of liquid fuels which are economically competitive with the remaining petroleum fuels which are still currently available. Traditionally, coal liquefaction has been performed at exceedingly high pressures and with the need for large quantities of expensive hydrogen. Additionally, in order to produce improved yields of liquid product from coal, expensive metal catalysts of the molybdenum, cobalt and nickel type in various physical forms have been utilized. All of these aspects of the previous attempts to provide commercially viable liquid fuels from coal have significantly affected the costs of producing such fuels.
Subsequently, attempts have been made to reduce the processing costs for the liquefaction of coal to liquid fuels. Generally, liquefaction pressures have been reduced from the 5,000 to 10,000 psi processing range to a range of 2,000 psi or less. Catalysts for the coal liquefaction have also been chosen for their lack of initial expense, as well as their selective activity for the liquefaction reactions.
In this light, U.S. Pat. No. 3,162,594 discloses the use of an inexpensive disposable catalyst, such as red mud, in hydrogenating coal extract. The spent catalyst from a coal extract hydrogenator is recovered by conventional solid/liquid separation and is recycled to a coal extract hydrogenator either without any treatment or after regeneration. Furthermore, U.S. Pat. No. 3,162,594 discloses the recycling of a spent supported catalyst from a downstream hydrocracker, after crushing, to a coal extract hydrogenation zone. Coal extract is the material obtained by the solvent extraction of coal after being separated from the mineral matter and the undissolved coal. It contains a minute, unfilterable amount of metallic contaminants, commonly referred to as ash. This recycle concept has been used not only for catalysts, but also for the solvent for a coal liquefaction reaction as taught in U.S. Pat. No. 3,188,179.
Recycling spent catalysts diminishes the catalyst expense, but involves a reduction in catalyst activity. Various techniques have been utilized to improve recycle catalyst activity, and exemplary of such techniques is U.S. Pat. No. 3,232,861 in which a supported catalyst from a coal extract hydrocracker is ground to expose previously unexposed internal surface area of the catalyst and to renew its activity before reintroducing the catalyst into a coal extract hydrotreater.
U.S. Pat. No. 3,488,279 discloses the recycle of a supported catalyst from a liquid coal product hydrocracker to a catalytic hydrogenation zone to hydrotreat coal extract before feeding it to a hydrocracker.
The recycle of catalyst in a coal liquefaction process is further described in U.S. Pat. Nos. 3,527,691 and 3,549,512 wherein the process is practiced in the absence of any substantial liquid phase and the catalyst functions as an adsorbent for the product of the coal conversion.
In U.S. Pat. No. 4,159,238, a coal liquefaction mineral residue and solid SRC are recycled from a downstream separator tower back to the coal liquefaction reactor.
U.S. Pat. No. 4,189,372 discloses the recycle of solvent from a coal extract hydrocracker back to the coal liquefier vessel.
The use of spent hydrotreating catalysts from the hydrogenation of petroleum, petroleum-derived liquids, Fischer-Tropsch liquids and shale oil hydrocarbon processes to a coal liquefaction process is described in U.S. Pat. No. 4,295,954. The recycled catalysts are taught to be selected from those catalysts used for the hydrogenation of high quality hydrocarbons.
Metal compounds such as oxides and sulfides of elements from Groups VI and VII of the Periodic Table are known to be good hydrogenation catalysts. Silica and alumina alone are also known to be good cracking catalysts in petroleum refining industry because of their acidity. It is also known that when metals from Group VI and VIII are deposited on either silica or alumina or both, they produce good hydrocracking catalysts.
The activity of any hydrocracking catalyst depends greatly on the metal loading, surface area and pore volume of the catalyst. If ash is not removed from coal extract prior to catalytic hydrocracking using a supported catalyst, the ash tends to deposit on the catalyst causing a reduction in surface area, as well as a reduction in the pore volume of the catalyst and eventually catalyst deactivation.
Coal extract is not similar to, nor does it behave similarly to other hydrocarbon materials, such as petroleum derived material, primarily because coal extract has a significantly different chemical structure from that of other hydrocarbon materials. Coal extract is solid at room temperature, whereas the petroleum-derived materials are liquid. Coal extract is rich in asphaltenes and preasphaltenes (high molecular weight compounds having low hydrogen content), while other hydrocarbonaceous materials contain small amounts of asphaltenes and do not contain preasphaltenes at all. In comparison with petroleum fuels and residue, coal liquids generally exhibit slightly higher carbon content, but significantly lower hydrogen content. The coal liquids have a higher degree of aromaticity and a more highly condensed ring structure than petroleum. A more striking difference between the coal liquids and petroleum fuels is the heteroatom content. Nitrogen and oxygen in coal liquids are much higher than in petroleum, but sulfur is somewhat lower. Coal extract ash is not similar nor does it behave similarly to ash contained in petroleum and other materials. The metallic contaminants, i.e., ash contained in petroleum derived liquids generally are associated with a porphyrin type of molecule which is to a large extent soluble in the petroleum. In contrast, up to 50 wt% of the metallic contaminants in coal extract are insoluble and finely divided particles. The metals in coal extract include Na, Si, Fe, Ca, Mg, Al, Ti, and Boron. The petroleum derived material contain predominantly Ni, Ti, and Vanadium.
It is known that coal extract readily undergoes degradation when it is subjected to thermal treatment. The degradation is manifested by the formation of coke, hydrocarbon gases and by the increase in the high molecular weight, hydrogen deficient portion of the extract. The benzene-insoluble (preasphaltene) content of the extract is a measure of this undesirable, high molecular weight extract portion.
The finely divided ash present in the coal extract can diffuse through the fine pore structure of a supported catalyst and deposit thereon during hydrocracking. This significantly reduces the surface area and pore volume of the catalyst. Metal deposition coupled with coke deposition drastically reduces the activity of the supported catalyst. Such decrease in activity forces resort to more frequent replenishment of the catalyst with fresh catalyst.
The prior art has made many attempts to provide an economic process for the liquefaction of coal to liquid fuel products. However, the full utilization of inexpensive catalyst material in a process for the production of liquid fuels from coal wherein the liquid fuels produced constitute a predominent portion of the product of the process has not been taught in the prior art. Such an advantage is realized in the process of the present invention as described below.